Hitherto, an air pollution control system for controlling the flue gas discharged from the boiler installed in thermal power plants and the like has been known. The air pollution control system includes a denitration device configured to remove nitrogen oxide from the flue gas emitted from the boiler, an air heater configured to recover the heat of the flue gas passing through the denitration device, a precipitator configured to remove the soot in the flue gas after heat recovery, and a desulfurization device for removing sulfur oxide in the flue gas after the dust removal. As the desulfurization device, a wet desulfurization device configured to remove the sulfur oxide in the flue gas by bringing a limestone absorbent or the like into gas-liquid contact with the flue gas has been generally used.
Waste water discharged from the wet desulfurization device (hereinafter, referred to as “desulfurization waste water”) contains various kinds of harmful substances, for example, ions such as chloride ions and ammonium ions, and mercury in a large quantity. For this reason, although it is necessary to remove these harmful substances from the desulfurization waste water before the desulfurization waste water is discharged to the outside of the system, there are problems in that the removal process of various kinds of harmful substances contained in the desulfurization waste water is complicated, and the processing cost is high. Therefore, in order to save the processing cost of the desulfurization waste water, a method of reusing the desulfurization waste water in the system without discharging to the outside of the system has been suggested. For example, Patent Literatures 1 and 2 disclose an air pollution control device having a configuration in which equipment branched from a flue of a main line, to which a denitration device, an air heater, a precipitator, and a desulfurization device are connected, to gasify the desulfurization waste water by spraying is separately installed, and after the harmful substances are precipitated by introducing a part of the flue gas from the flue gas duct of the main line into the equipment and spraying the desulfurization waste water in the flue gas in the equipment to evaporate the flue gas, the flue gas is returned to the flue gas duct of the main line (Patent Literatures 1 and 2).